I have watched two of my hens hatch chicks and both lost weight. The first one did moreso, because I read the same things you did and just assumed that's how it was to be. She had her own coop complete with water and food, and I'd say she lost a significant amount of weight. I even took her out each day for the first 10 days or so to get her to eat and poop. I always kept food and water in the coop with her.This all makes total sense to me. And I should say at the start that I have zero experience of broody hens. What I don't understand is why on BYC there are so many horror stories about hens wasting away sitting on nests for what sounds like weeks even when there aren't any eggs.
In nature if a bird loses its eggs (eg if a predator takes them) I believe the bird eventually abandons the nest and may even move on to create a new one.
So is all the fuss about broodiness on BYC because today's hens (not @Shadrach 's obviously) are bred to have lost that instinct to move on when a nest isn't viable? Or is it over-anxious chicken-keepers?
But, this time, with Surly, I didn't do that. Surly came out on her own each day for the first couple weeks. Then, when she decided to stay in, I didn't take her out. Again, I had food and water in the coop with her, a small 4x4 coop with its own run, intended for my broodies. This time I actually would bring some grains and put them right in front of her on the straw each day, just about a tablespoon full. That way if she didn't get up to eat, I knew she would be eating something. She didn't come out at all the last week, and would just move her eggs to another part of the coop if she left a poop, and I would clean that up whenever it happened. I kept a close eye on her.
All that to say, she did not lose as much weight as Shirl did. In fact, it didn't look like she lost any weight at all. I could only tell by picking her up.
Poor Shirl. Neither she nor I had any experience to draw from.
So, as a novice at this business of allowing hens to raise their own chicks, I'd say it's the conditions. An outside wild hen will find some food right around her nest, bugs, foliage, etc. Around here she would get dew in the mornings for water. Not sure about the drier climates.